Fig. 2-21. Pulse width and PRF effects on pulse modulated spectrum.
Identification and Frequency Measurement of
Displayed Signals
Bandpass and dispersion characteristics of spectrum an-
alyzers require very limited preelection ahead of the first
mixer. Signals with frequencies different than that indicated
by the dial will therefore appear on the display. These
signals are called spurious responses. See spurious responses
under Spectrum Analyzer Terms at the beginning of this sec-
tion.
The dial scales for the Type 491 indicate frequencies that
are below the local oscillator frequency by the IF (200 MHz).
For example: a dial reading of 700 MHz means the local
oscillator frequency of 900 MHz (700 MHz + 200 MHz). This
local oscillator frequency will mix with 700 MHz and 1100
MHz to produce the IF of 200 MHz. The 1100 MHz response
is called the image. Note that the image response is twice
the IF away from the true, or dial calibrated response. Har-
monics of the local oscillator fundamental frequency also
convert incoming signals to the IF response. For example,
the second harmonic of 900 MHz (1800 MHz] will mix with
1600 MHz and 2000 MHz.
These response are identified and read as follows:
1. Tune the RF Center Frequency across a dispersion win-
dow and observe the signal movement.
2. True responses move across the dispersion window from
left to right, on the Type 491, as the RF CENTER FREQUENCY
is increased, or in the same direction as the tuning knob
is turned.
3. Images move across the dispersion window opposite to
the direction of the true response.
4. IF feedthrough signals are not tunable and remain fixed
in position as the RF CENTER FREQUENCY is tuned.
5. Signal frequency shifts across the dispersion window that
are not coincident with the RF CENTER FREQUENCY change
are spurious.
Some of these spurii are mixing with higher
harmonics of the local oscillator. The upper dial scales of
the Type 491 are calibrated to harmonics of the local os-
2-19
Summary of Contents for 491
Page 4: ...i i Fig 1 1 The Type 491 Spectrum Analyzer ...
Page 24: ...2 16 Fig 2 17 Control set up chart ...
Page 34: ...Fig 3 1 Function block diagram of the Type 491 3 2 ...
Page 42: ...Fig 3 10 Block diagram of the video detector and vertical amplifier 3 1 0 ...
Page 48: ......
Page 53: ...Fig 4 4 Power Supply Circuit board assembly with wiring color code 4 5 ...
Page 54: ...Fig 4 5 Horizontal Display circuit board assembly showing color code to pin connectors 4 6 ...
Page 55: ...Fig 4 6 IF Control board assembly Wiring color code to pin connector 4 7 ...
Page 64: ...Fig 4 20A Tube subassembly removal procedure 4 16 ...
Page 65: ...Fig 4 20B Tube subassembly installation procedure 4 17 ...
Page 68: ...Fig 4 22 Power supply board assembly with component call out 4 2 0 ...
Page 69: ...Fig 4 23 Vertical Amplifier and Blanking board assembly with component call out 4 21 ...
Page 70: ...Fig 4 24 IF control board assembly with component call out 4 22 ...
Page 71: ...Fig 4 24 IF control board assembly with component call out 4 23 ...
Page 72: ...Fig 4 25 Horizontal display board with component call out 4 24 ...
Page 73: ...Fig 4 25 Horizontal display board with component call out 4 25 ...
Page 74: ...Fig 4 26 Phase lock board with component call out 4 26 ...
Page 88: ......
Page 90: ...Fig 6 1 Test equipment recommended for calibration of the Type 491 6 2 ...
Page 138: ......
Page 192: ......
Page 195: ...SECTION II B 3 ...
Page 196: ......
Page 200: ......
Page 201: ...SECTION 9 DIAGRAMS MECHANICAL PARTS LIST ILLUSTRATIONS ACCESSORIES ...
Page 202: ......
Page 203: ...9 1 ...
Page 204: ...9 3 ...
Page 205: ...9 5 ...
Page 206: ...9 7 ...
Page 207: ...9 9 ...
Page 208: ...9 11 ...
Page 209: ...9 13 ...
Page 210: ...9 15 ...
Page 211: ...9 17 ...
Page 212: ...9 19 ...
Page 213: ...9 21 ...
Page 214: ...9 23 ...
Page 215: ...9 25 ...
Page 216: ...9 27 ...
Page 217: ...FIG 1 9 29 ...
Page 218: ...FIG 2 REAR 9 31 ...
Page 219: ...9 33 FIG 3 IF CHASSIS PHASE LOCK ASSEMBLIES ...
Page 220: ...FIG 4 POWER CHASSIS 9 35 ...
Page 221: ...FIG 5 TIME DIV SWITCH OSCILLATOR ASSEMBLIES 9 37 ...
Page 222: ...FIG 6 CRT SHIELD ASSEMBLY 9 39 ...
Page 223: ...FIG 7 CABINET ASSEMBLY HANDLE 9 4 1 ...
Page 224: ...FIG 8 491 STANDARD ACCESSORIES FIG 8 491 STANDARD ACCESSORIES 9 43 ...
Page 225: ......
Page 226: ...PIN 028017 000 ...